PSI - Issue 47

Ilia Nikitin et al. / Procedia Structural Integrity 47 (2023) 617–622 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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The figure 4 shows the VHCF crack shape for the following distances of hard element location from the specimen surface: near the surface (Fig.4-a), 200 micrometers (Fig.4-b), and 400 micrometers (Fig.4-c). In the case of near to surface location the macro crack is being on the plane of maximum normal stress. A short crack was obtained in the second plane of maximum normal stress. In the case of deeper location of artificial defect, a clear double-cross crack was obtained but the initial stage is not associated with shear mechanisms. When the distance from the surface is large enough a double-crossed crack (X-type) is formed with clear stage of crack growth due to shear. Such spatial shape of VHCF crack is similar to experimental one, Fig.4-d, when the crack initiated at the distance about 300 – 400 micrometers, Fig.2-b. The results of numerical simulation are close to experimental shapes of VHCF torsion cracks and predicts the spontaneous bifurcation of the main crack opening mechanisms. Therefore, the proposed multi-regime model is a powerful instrument to predict the spatial crack growth under complex multi-axial loading. 4. Conclusions The results of VHCF torsion tests shows two types of crack initiation: (1) surface and (2) subsurface. In the case of surface crack initiation, the scenario of crack developing is similar to HCF regime. In the case of subsurface crack initiation , the scenario is different and ‘manufactory roof’ morphology is absent. The subsurface crack initiation is not associated to any metallurgical defects of Ti-alloy like inclusion, pore, or porosity. The subsurface crack origins in the bulk of typical material structure. The subsurface crack provocative the secondary surface cracks that lead to X-type macro crack formation. The multi-regime two-criteria fatigue failure model was successfully applied to predict the spatial crack path shape in smooth specimens. The numerical simulation on VHCF torsion specimen shows the crack initiation by shear mechanisms and further bifurcation to crack propagation on plane of maximum tensile stress with associated normal crack opening mechanism. The modal is capable to reproduce all key features of torsion crack developing. The numerical simulation on smooth specimen with hard artificial particle shows a depth-depending behavior of the macro crack. In the case of close-to-surface location of the hard particle a quasi-single crack was observed. With increasing the depth of hard particle location into the bulk of material a double-cross or X-type crack was obtained. When the hard particle was placed at distance of 100 micrometers the crack driving by shear mechanism was not observed. With increase the distance to 200 – 400 micrometer a clear part of the crack developed by shear mechanism was obtained. The results of numerical simulation are in good agreement with the fractography analysis. Acknowledgements This work is realized with financial support of Russian Science Foundation (Project № 19-19-00705). References Bathias, C., Paris, P., 2004. Gigacycle fatigue in mechanical practice, Dekker, New York, p. 328. Nikitin, A., Palin-Luc T., Shanyavskiy A., 2016. Crack initiation in VHCF regime on forged titanium alloy under tensile and torsion loading modes. International Journal of Fatigue, Vol. 93, pp. 318-325. Nikitin, A., Palin-Luc, T., Shanyavskiy, A., Bathias, C. 2016. Comparison of crack paths in a forged and extruded aeronautical titanium alloy loaded in torsion in the gigacycle fatigue regime, Engineering Fracture Mechanics, 167, pp. 259-272. Nikitin, I.S., Burago, N.G., Nikitin, A.D., Stratula, B.A. 2021. Multi-Mode Model and Calculation Method for Fatigue Damage Development, Applied Mathematics and Computational Mechanics for Smart Applications, Vol. 217, pp. 157 - 170. Nikitin, I.S., Burago, N.G., Nikitin, A.D. 2022. Damage and Fatigue Fracture of Structural Elements in Various Cyclic Loading Modes// Mechanics of Solids. Vol. 57. No. 7. Pp. 1793 – 1803.